Linear-beam microwave tubes such as traveling-wave tubes (acronym TWT) or klystrons essentially comprise an electron gun having a cathode providing a cylindrical beam of electrons in an evacuated cylindrical envelope of a microwave structure of the tube. A collector, at one end of the microwave structure, gathers the electrons of the beam output by the cylindrical envelope.
The electrons output by the cathode are focused in the form of a linear beam in the evacuated cylindrical envelope by means of a magnetic field. This magnetic field may be created either by permanent magnets, or by windings around the evacuated cylindrical envelope.
The microwave structure is the element of the tube where an interaction takes place between the electron beam and an electromagnetic wave which may be, either applied to a radiofrequency input (RF) of the tube in the case of amplifier tubes, or created in the tube in the case of tubes operating as microwave oscillators. More precisely the beam of electrons yields part of its kinetic energy to the electromagnetic wave in the microwave structure.
The microwave structure comprises resonant cavities and drift tubes in the case of a klystron and of a helix or coupled cavities in the case of a TWT.
The vacuum inside an electron tube is never perfect and gas molecules present in the evacuated envelope of the tube pass into the beam and lose electrons (phenomenon of ionization) under the impact of the electrons of the beam which are very energetic (typically several Kev). Positive ions are thus formed in the beam. The positive charges being attracted by the negative charges of the beam, the positive ions remain locked in the beam in a position of radial equilibrium.
FIG. 1 shows an axial portion of a helix amplifier TWT of the state of the art.
The TWT of FIG. 1 comprises a microwave structure 10 having, along a longitudinal axis ZZ′ of the tube, an evacuated envelope 12 containing a helix 14 traversed along this axis ZZ′ by a cylindrical electron beam 16 propagating from the cathode to the anode of the tube. The direction of propagation of the beam is represented by the arrows B in FIG. 1.
The microwave structure 10 comprises, in a known manner, permanent magnets separated by magnetic spacers (not represented in the figure) so as to provide a confinement field for focusing the beam 16 on the axis ZZ′ of the evacuated envelope. An RF input ensures an RF connection of the helix of the TWT with for example an external RF source.
As has been described above, the gas molecules passing into the electron beam that are struck by electrons of said beam produce positive ions 20 which move slowly, for example, from the cathode (not represented in FIG. 1) side of the tube.
The axial force acting on these positive ions 20 is very weak and they may remain in the beam 16 for a very long time before their slow drift velocity removes them, either toward the cathode, or toward the collector of the tube. Consequently, a large quantity of ions may accumulate inside the beam 16 and generate a significant positive space charge that may compromise good focusing of the beam. This concentration of positive ions in the beam results in a periodic focusing instability, called ion relaxation. This is a nuisance phenomenon which disturbs the RF telecommunication signal, for example in the case of an amplifier of TWT type, and which one seeks to eliminate.
The positive ions produced by the impacting of the electrons of the beam with the gas molecules in the evacuated envelope, in addition to the ion relaxation phenomenon, present another drawback. Indeed, when these ions arrive, after their long journey in the beam, level with the cathode of the tube, the negative potential of the cathode attracts them, producing impacts on the cathode and a deterioration of its emissive surface by a phenomenon of ion erosion or “sputtering”.
To eliminate the impact of the positive ions on the cathode of the tube, state-of-the-art electron tubes are equipped with a device designated by the term “ion barrier”. The ion barrier is an electrode placed after the cathode and carried to a positive potential so as to repel or reflect the positive ions originating from the beam. The drawback of the ion barrier is that it aggravates the ion relaxation described above, disturbing yet more strongly the RF signal in the tube. Indeed the positive ions can no longer be removed via the cathode and spend more time in the electron beam.